The present invention relates to wholly aromatic copolyesters, particularly wholly aromatic copolyesters with controlled flexibility.
Aromatic copolyesters which are para-linked are known to feature thermotropic behavior in which heating the semicrystalline solid results in melting to a mesomorphic phase, often nematic, giving rise to desirable molding characteristics, mechanical properties, and solvent resistance. Often, their high modulus and strength come at the expense of low toughness, manifested as failure strains less than 5%. In addition, such polymers feature nematic-isotropic transition temperatures in excess of thermal decomposition, eliminating the potential benefit of traversing the isotropic-nematic phase transition during processing.
Since thermotropic liquid crystallinity in polymers was first discovered in the 1970""s, thermotropic liquid-crystalline polymers (LCPs) have been predominantly aromatic polyesters. The two major commercial materials are wholly aromatic polyesters made from diacids (AA) and diols (BB), as well as from AB-type monomers, such as 2-hydroxy, 6-carboxylic acid naphthalene. These wholly-aromatic, para-linked, polyesters have found usage primarily as molding resins with mechanical properties approaching those of polymer matrix composites. In addition, fiber and film products are beginning to emerge.
Despite the success of thermotropic LCP technology, there are several significant challenges to be addressed. The first of these is the large disparity in mechanical properties between molding (30 ksi strength) and fiber spun (400 ksi strength) articles. Second is the extreme sensitivity of mechanical properties to the conditions of melt processing, such as extrusion speeds, temperature profile, and die design.
We have synthesized LCPs characterized by accessible nematic-isotropic transition temperatures leading to two effects. First, the nematic defect history can be xe2x80x9cerasedxe2x80x9d to obtain a reproducible starting morphology for processing. Second, the flow-induced isotropic-nematic transition can be exploited to obtain outstanding orientational order and mechanical properties.
Accordingly, it is an object of the present invention to provide novel liquid-crystalline polymers.
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the present invention, there is provided a wholly aromatic copolyester having repeating units of the formula: 
wherein Ar1 is 
wherein Q is selected from the group consisting of xe2x80x94H, xe2x80x94CH3, xe2x80x94CF3, xe2x80x94Cl, xe2x80x94Br, and xe2x80x94C6H5;
wherein Ar2 is selected from the group consisting of 
wherein X is selected from the group consisting of xe2x80x94Br, xe2x80x94Cl, xe2x80x94CH3 and xe2x80x94C6H5;
wherein Ar3 is 
wherein Ar4 is selected from the group consisting of 
and
wherein m has a value of 0.05 to 0.95 and n has a value of 100-m.
The wholly aromatic copolyesters of this invention are prepared by the condensation copolymerization of a para-linked dicarboxylic acid of the formula HOOCxe2x80x94Ar1xe2x80x94COOH (hereinafter referred to as A), a para-linked diol of the formula HOxe2x80x94Ar2xe2x80x94OH (hereinafter referred to as B), and a dicarboxylic acid of the formula HOOCxe2x80x94Ar3xe2x80x94COOH (hereinafter referred to as C), wherein Ar1, Ar2 and Ar3 are as defined above. The polymerization method includes, but is not limited to, polymerization methods known in the art, for example, the melt polymerization method or solution polymerization method can be used for synthesis.
In the case where the copolyesters of the present invention are produced by solution polymerization, for example a predetermined amount of each of the HOOCxe2x80x94Ar1xe2x80x94COOH, HOxe2x80x94Ar2xe2x80x94OH, and HOOCxe2x80x94Ar3xe2x80x94COOH monomers are dissolved in a solvent and heated. Alternatively, the monomers are dissolved in an acid acceptor such as pyridine and heated in the presence of aryl sulfonyl chloride/dimethylformamide or diphenyl chlorophosphateldimethylformamide whereby the desired copolyester can be easily obtained.
The charging ratio (molar ratio) of the A, B and C monomer components in synthesis of the copolyesters of the present invention by solution polymerization is as follows. The sum of A+C=B, and the ratio A:C can range from 5:95 to 95:5, preferably 10:90 to 90:10, more preferably 80:20 to 60:40.
The solvent used for solution polymerization includes, but is not limited to, halogen based solvents such as o-dichlorobenzene, dichloroethane, tetrachloroethane and the like, polar solvents such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), N-methylpyrrolidone (NMP) and the like, and ether based solvents such as tetrahydrofuran (THF), dioxane and the like. The acid acceptor includes, but is not limited to, pyridine, triethylamine, tripropylamine and the like. Although the reaction conditions for solution polymerization are not particularly limited, the temperature is usually 50xc2x0 to 200xc2x0 C., preferably 60xc2x0 to 150xc2x0 C., and the reaction time is usually 1 hour or more, preferably about 2 hours to 10 hours.